Comparisons of mesoscale variability in the Semtner‐Chervin 1/4° model, the Los Alamos Parallel Ocean Program 1/6° model, and TOPEX/POSEIDON data

Abstract

Measures of mesoscale variability in the Semtner‐Chervin 1/4° and the Los Alamos Parallel Ocean Program (POP) 1/6° models were compared with those obtained from TOPEX/POSEIDON (T/P) data. The objectives of these comparisons were two‐fold: the first was to validate the models using altimetry as a measure of the variability of the real ocean, and the second was to evaluate the effect of increased model resolution/decreased horizontal friction. Mesoscale root‐mean‐square (rms) sea surface height (SSH), eddy kinetic energies, and length scales were used to quantify the mesoscale variability. Results showed that the models reproduced the distribution and much of the magnitude of this variability associated with the major current systems; however, in the oceans' interiors the magnitude was underrepresented. The 1/6° and 1/4° models were found to explain about 60% and 50% of the global T/P variability, respectively. Estimates of eddy kinetic energy (and rms velocities) from T/P and the models were compared, demonstrating that the models were less energetic than the T/P fields. Independent comparisons were made with lagrangian drifters in the Pacific basin. Excellent agreement was found between the total POP velocity fields and the drifter data in the tropics, where the T/P geostrophic values were too high due to error amplification by the 1/ƒ factor. In the midlatitudes, the drifter values exceeded those derived from the total model velocities; the T/P results lay between the two. Differences are attributed to the drifter analysis choices and possible residual noise in the altimetry data. The effect of increased resolution/decreased friction was best seen in the length scales where the POP scales agreed more closely with the T/P values than with the 1/4° model The distribution and magnitude of the POP length scales were generally in agreement with the T/P values between 10° and 40°. Near the equator, discrepancies were due to the long equatorial and instability waves, whose long wavelengths were too great to be resolved by the the combination of noise in the altimeter slopes, and the particular definition of length scale chosen.